Ayke van Laethem

she/her

Bare metal programming from the ground up

@ayke@hachyderm.io
@aykevl
@aykevl

Who am I

• MicroPython:
    worked on nrf5x support long ago
• TinyGo:
    creator and maintainer
• Maker of blinky jewelry

Topics for this talk

• How to read datasheets
• Linker scripts
• Interrupt vectors
• Blinking LEDs

"Hello world"

Blink LED

Turn on LED!

Read the manual!

"Hello world"

#include <stdint.h>

// Memory mapped I/O (values from the datasheet).
#define PORTB *(volatile uint8_t*)(0x23)
#define DDRB  *(volatile uint8_t*)(0x24)

int main() {
    DDRB  |= 1; // set pin PB0 as an output
    PORTB |= 1; // set pin PB0 high

    // wait forever
    while (1) {
    }
    return 0;
}
					

sbi 0x04, 0   ; set bit 0 in DDRB  (0x24)
sbi 0x03, 0   ; set bit 0 in PORTB (0x23)
rjmp .-2      ; jump back one instruction (loop)
					

Reset vector

Reset vector

"The lowest addresses in the program memory space are by default defined as the Reset and Interrupt Vectors."

Translation: the reset handler and the interrupts are defined at the beginning of memory, probably at address zero.

Reset vector

"[...] the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt handling routine, [...]"

Translation: the chip jumps to the location in memory where the interrupt is defined.

Program memory

Interrupt vector table

jmp __vector_RESET         ; address 0
jmp __vector_INT0          ; address 4
jmp __vector_INT1          ; address 8
jmp __vector_PCINT0        ; address 12
jmp __vector_PCINT1        ; address 16
jmp __vector_PCINT2        ; address 20
jmp __vector_WDT           ; address 24
jmp __vector_TIMER2_COMPA  ; address 28
jmp __vector_TIMER2_COMPB  ; address 32
jmp __vector_TIMER2_OVF    ; address 36
; etc
					

...

Questions?

How to find me:
@ayke@hachyderm.io
@aykevl
@aykevl